Solid dosage forms such as tablets, pellets, capsules and the like are covered by a coating if required. The reasons why solid dosage forms may be covered by a coating are manifold. For instance, a coating may permit to provide a solid dosage form which is resistant to gastric juice. Further, a coating may provide the possibility to modify a release of an ingredient comprised in the solid dosage form. For example, the release of the ingredient comprised in the solid dosage form may be retarded or delayed with respect to an uncoated solid dosage form. Additionally, a coating may prevent that the solid dosage form is damaged. Also, a coating may reduce or even prevent that an ingredient or a substance of a solid dosage form undergoes a chemical reaction, for example, due to contact with air, humidity or a chemical substance. Moreover, some ingredients of a solid dosage form may have a displeasing taste which may be masked by a coating. Further, coatings may also be used in order to maintain an expiration date of a solid dosage form. In order to fulfill these requirements, the coating needs to be applied with a certain thickness, homogeneity and/or quality. For example, if a coating layer is not thick enough, the coating may crack upon contact with another solid dosage form or a container or the solid dosage form may not be resistant to gastric juice.
In order to determine a thickness of a coating, the thickness of the coating may be measured after the coating process is finished, for example by weighing the solid dosage form before and after the coating is formed on the solid dosage form and determining the percentage of the weight gain. Alternatively, the thickness of a coating on a solid dosage form may be determined with a spectroscopy method, for example near infrared or Raman spectroscopy. Although near infrared or Raman spectroscopy may be applied during a coating process, these methods need a reference model which allows to connect a measured spectrum to a corresponding coating thickness. For example, a coating thickness corresponding to a certain spectrum may be determined using a scanning electron microscope which, however, may destroy the coating and/or the solid dosage form, particularly chemical compounds in the coating and/or the solid dosage form. Further method for determining a thickness of a coating may be terahertz pulse imaging, magnetic resonance imaging (MRI) or X-ray microcomputed tomography (XμCT). In particular, the methods know in the prior art may not allow to determine other properties of the coating than the thickness of the coating. Further, the known methods may be too slow and/or not precise enough to be applied during a process forming the coating.